Reddit reviews Hot Springs and Hot Pools of the Northwest: Jayson Loam's Original Guide (Hot Springs & Hot Pools of the Northwest: Jayson Loam's)

The only other official resort in Idaho is Sun Meadow in Worley, 260 miles from Boise. Oregon has a few, but I don't know if any are in the Eastern half, so it depends on how far you're willing to drive. Utah has none, haven't heard of any in Nevada. Don't know about the other neighboring states.

Idaho's big advantage is the hot springs. It's known for having some of the best hot springs in the country, and many of them are traditionally clothing optional. The trick is that none of them, to my knowledge, are officially clothing optional, so it takes familiarity and experience to know which ones are nude-friendly in practice and which are not. I have two books, Hiking Hot Springs in the Pacific Northwest and Hot Springs and Hot Pools of the Northwest, that have extensive information on the whole region, including notes on the trend of clothed vs nude at each one, but I take that with a grain of salt given that it's been a few years since the most recent editions. I can't advise on any of the Idaho springs, since I've really only gone to one in Oregon. It's closer than any of the Idaho springs, but it's a mixed bag at best when it comes to nudity.